The present invention relates generally to magnetic flowmeters. More specifically, the present invention relates to an improved H-bridge constant current driver design for a magnetic flowmeter.
A magnetic flowmeter measures the volumetric flow rate of a conductive fluid by detecting the velocity of the fluid passing through a magnetic field. Magnetic flowmeter systems typically include a flowtube assembly and a transmitter assembly. The flowtube assembly is installed in a process piping line, either vertically or horizontally, and includes a pipe section, a coil section, and electrodes. The coils are located on opposite sides of a cross section of the pipe. The coils, energized by a coil drive current from the transmitter, develop a magnetic field along the cross-section of the pipe. Two electrodes are located across the pipe from each other along a line which is perpendicular to the magnetic field. Fluid passing through the pipe is electrically conductive. As a result of the conductor movement through the magnetic field, an electric potential or electromotive force (EMF) is induced in the fluid which is detected by the electrodes. Operation is thus based on Faraday's law of electromagnetic induction.
Magnetic flowmeters are often used in applications where mixtures run through the flowtube. For example, in the paper industry, pulp, various chemicals and water run through the flowtube. In applications such as this, as the consistency of the fluid moving through the flowtube increases from a nominal value of 3% or 4% to a higher consistency of 10% to 15%, the paper pulp moving through the tube causes process noise to increase. As process noise increases, the signal-to-noise ratio decreases, which decreases the accuracy of the magnetic flowmeter output. To reduce the effects of process noise, attempts have been made to increase the magnetic field strength by increasing the coil drive current, which correspondingly increases the EMF induced in the fluid. Increasing the EMF induced in the fluid results in an increased signal-to-noise ratio and measurement accuracy.
Because the power dissipated in the magnetic flowmeter current driver and flowtube coils depends on the square of the current produced by the current driver, improving the signal-to-noise ratio by increasing the current results in substantially increased power dissipation. The increased power dissipation is a difficult design limitation. Therefore, an energy efficient AC current driver is needed to drive the flowtube coils at the increased current level.